This invention relates to on a process for preparing microcrystalline perborate products and highly concentrated, storage stable aqueous solutions which are used for said preparation in one variant of the process and which contain a boron-oxygen compound, hydrogen peroxide, and sodium hydroxide in a special composition.
In the production of liquid bleach, disinfectant and liquid detergent formulations, including liquid aqueous detergent formulations, producers wish to incorporate compounds having a bleaching activity such as perborates (e.g. sodium perborate tetrahydrate=PBS4) in the liquid formulations. The perborates exist as suspended particles in these formulations and therefore must not exceed a certain particle size for reasons relating to the physical stability of the suspension. Therefore, attempts are made to incorporate the perborates (e.g. PBS4) in the formulations in a form which is as finely divided as possible, i.e. in the form of microparticles having a particle diameter not greater than 20 micrometers, preferably with a particle diameter of less than 5 micrometers.
According to the processes of the prior art, it is difficult to prepare sufficiently fine perborate particles which are highly suitable e.g. for forming a suspension in liquid detergent formulations. In order to ensure the physical stability of the suspension of perborate particles in liquid detergent formulations, thickeners and dispersing aids must be used in the prior art. It is however, disadvantageous that, as a result of this measure, the proportion of active ingredients in the liquid detergent formulation is decreased by the proportion of thickeners and dispersing agents.
Aqueous liquid detergent formulations containing sodium perborate tetrahydrate in the form of fine crystals, and processes for forming the perborate in the form of fine crystals in such formulations are already known from European patent application No. EP 294,904. An "in situ crystallization" of PBS4 is described, i.e. the crystallization of PBS4, optionally after in situ formation from metaborate and H.sub.2 O.sub.2 in the presence of at least 5% by wt. surfactant and at least 5% by wt. builder. Moreover, liquid aqueous washing agent formulations are claimed which contain PBS4 "crystallized in situ" having particle diameters m of 0.5 to 20 micrometers. According to the examples, the particle sizes obtained at the lower end of the range average from 4 to 7 micrometers.
The above-mentioned European patent application discloses two process variants for preparing these washing agent formulations which contain PBS4 "crystallized situ". The first process variant is a dissolution/recrystallization process in which the perborate particles are formed by intensively stirring a perborate composed of relatively large crystals in the presence of the surfactant and builder. PBS4 particles having an average particle diameter of approximately 7 micrometers are obtained. The second process variant is a process in which the perborate is formed in situ by chemical reaction of metaborate (Na:B=1:1) with hydrogen peroxide in the presence of at least 5% by wt. surfactant and at least 5% by wt. builder and crystallized out. In this process, either metaborate is used as the boron component, or it is also possible to use borax or boric acid. In this case, however, metaborate is formed initially due to the stoichiometric addition of NaOH, and the metaborate is subsequently converted by the addition of H.sub.2 O.sub.2 in the presence of surfactant and builder to the perborate which crystallizes out. PBS4 particles having an average particle diameter of approximately 4 micrometers are obtained.
The prior art processes have a number of disadvantages. The limited solubility of metaborate permits only moderate concentrations of dissolved boron components (approximately 20% by wt. metaborate in water at 20.degree. C.). Moreover, the only moderately concentrated metaborate solutions tend to crystallize prematurely.
In the process of European patent application no. EP 294 904, the presence of surfactant and builder is essential to obtain small particle diameters, but on the other hand, it can lead to undesired secondary reactions during the reaction with H.sub.2 O.sub.2. Moreover, there are problems with "in situ crystallization" with regard to control and reproducibility of the required small particle diameters.